diff --git a/wp-includes/classes.php b/wp-includes/classes.php
index 03c3e0c867..dc031e0b68 100644
--- a/wp-includes/classes.php
+++ b/wp-includes/classes.php
@@ -3,7 +3,7 @@
 class WP {
 	var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'debug', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots');
 
-	var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'what_to_show', 'showposts', 'nopaging', 'post_type', 'post_status');
+	var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'what_to_show', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and');
 	var $extra_query_vars = array();
 
 	var $query_vars;
@@ -17,6 +17,10 @@ class WP {
 		$this->public_query_vars[] = $qv;
 	}
 
+	function set_query_var($key, $value) {
+		$this->query_vars[$key] = $value;
+	}
+
 	function parse_request($extra_query_vars = '') {
 		global $wp_rewrite;
 
diff --git a/wp-includes/query.php b/wp-includes/query.php
index 6c0ad9c2f5..e76472f45d 100644
--- a/wp-includes/query.php
+++ b/wp-includes/query.php
@@ -413,6 +413,12 @@ class WP_Query {
 				$array[$key] = '';
 		}
 
+		$array_keys = array('category__in', 'category__not_in', 'category__and');
+		
+		foreach ( $array_keys as $key ) {
+			if ( !isset($array[$key]))
+				$array[$key] = array();
+		}
 		return $array;
 	}
 
@@ -548,6 +554,26 @@ class WP_Query {
 				$this->is_category = true;
 			}
 
+			if ( !is_array($qv['category__in']) || empty($qv['category__in']) ) {
+				$qv['category__in'] = array();
+			} else {
+				$qv['category__in'] = array_map('intval', $qv['category__in']);
+				$this->is_category = true;	
+			}
+
+			if ( !is_array($qv['category___not_in']) || empty($qv['category__not_in']) ) {
+				$qv['category__not_in'] = array();
+			} else {
+				$qv['category__not_in'] = array_map('intval', $qv['category__not_in']);
+			}
+
+			if ( !is_array($qv['category__and']) || empty($qv['category__and']) ) {
+				$qv['category__and'] = array();
+			} else {
+				$qv['category__and'] = array_map('intval', $qv['category__and']);
+				$this->is_category = true;	
+			}
+
 			if (  '' != $qv['tag'] )
 				$this->is_tag = true;
 
@@ -842,39 +868,49 @@ class WP_Query {
 		} else {
 			$q['cat'] = ''.urldecode($q['cat']).'';
 			$q['cat'] = addslashes_gpc($q['cat']);
-			$join = " LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
 			$cat_array = preg_split('/[,\s]+/', $q['cat']);
-			$in_cats = $out_cats = array();
-			$include_cats = $exclude_cats = '';
 			foreach ( $cat_array as $cat ) {
 				$cat = intval($cat);
 				$in = ($cat > 0);
 				$cat = abs($cat);
 				if ( $in ) {
-					$in_cats[] = $cat;
-					$in_cats = array_merge($in_cats, get_term_children($cat, 'category'));
+					$q['category__in'][] = $cat;
+					$q['category__in'] = array_merge($q['category__in'], get_term_children($cat, 'category'));
 				} else {
-					$out_cats[] = $cat;
-					$out_cats = array_merge($out_cats, get_term_children($cat, 'category'));
+					$q['category__not_in'][] = $cat;
+					$q['category__not_in'] = array_merge($q['category__not_in'], get_term_children($cat, 'category'));
 				}
 			}
-			if ( ! empty($in_cats) ) {
-				$include_cats = "'" . implode("', '", $in_cats) . "'";
-				$include_cats = " AND $wpdb->term_taxonomy.term_id IN ($include_cats) ";
-			}
+		}
 
-			if ( !empty($out_cats) ) {
-				$ids = get_objects_in_term($out_cats, 'category');
-				if ( is_array($ids) && count($ids > 0) ) {
-					$out_posts = "'" . implode("', '", $ids) . "'";
-					$exclude_cats = " AND $wpdb->posts.ID NOT IN ($out_posts)";
-				}
-			}
-			$whichcat = " AND $wpdb->term_taxonomy.taxonomy = 'category' ";
-			$whichcat .= $include_cats . $exclude_cats;
+		if ( !empty($q['category__in']) || !empty($q['category__not_in']) || !empty($q['category__and']) ) {
 			$groupby = "{$wpdb->posts}.ID";
 		}
 
+		if ( !empty($q['category__in']) ) {
+			$join = " LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
+			$whichcat .= " AND $wpdb->term_taxonomy.taxonomy = 'category' ";
+			$include_cats = "'" . implode("', '", $q['category__in']) . "'";
+			$whichcat .= " AND $wpdb->term_taxonomy.term_id IN ($include_cats) ";
+		}
+
+		if ( !empty($q['category__not_in']) ) {
+			$ids = get_objects_in_term($q['category__not_in'], 'category');
+			if ( is_array($ids) && count($ids > 0) ) {
+				$out_posts = "'" . implode("', '", $ids) . "'";
+				$whichcat .= " AND $wpdb->posts.ID NOT IN ($out_posts)";
+			}
+		}
+
+		if ( !empty($q['category__and']) ) {
+			$count = 0;
+			foreach ( $q['category__and'] as $category_and ) {
+				$join .= " LEFT JOIN $wpdb->term_relationships AS tr$count ON ($wpdb->posts.ID = tr$count.object_id) LEFT JOIN $wpdb->term_taxonomy AS tt$count ON (tr$count.term_taxonomy_id = tt$count.term_taxonomy_id) ";
+				$whichcat .= " AND tt$count.term_id = '$category_and' ";
+				$count++;
+			}
+		}
+
 		if ( '' != $q['tag'] ) {
 			$reqtag = is_term( $q['tag'], 'post_tag' );
 			if ( !empty($reqtag) )